On an outdoor grill, get enough charcoal lit to have a single layer coals below the meat. At the same time, soak a few handfuls of mesquite chips in water.

In making the filling, break the garlic head apart, then squeeze each soft garlic clove from its skin. In a small skillet or frying pan, heat one tablespoon of the olive oil. Add the garlic, and mash it into the oil with a fork until you form a rough paste.

Stir in the scallions, and cook for a minute or two, until the onions are soft. Remove the pan from the stove top, and let the filling mixture cool while you get the meat ready.

Trim off any fat from the sirloin, and cut a horizontal slit along the length of the steak to make a pocket. Be sure not to cut through to the other side. Spread the remaining 1/2 tablespoon of olive oil thinly over the sirloin.

After allowing the charcoal to become gray, spread the soaked mesquite chips on top of the coals. Lay the steak on the grill over the coals. Cover the meat with the grill lid, or make a foil tent to lay over the sirloin.

Grill the meat on one side for about 15 minutes, then turn it. Grill for another 10 to 15 minutes, until reaching your desired doneness. Serve immediately.